The ultimate goal of this project is to make possible a molecular approach to cancer prophylaxis and therapy for some types of viral neoplasia, especially human leukemia and cancer of the breast where retrovirus information has been detected. Some of our major findings resulting from our research efforts are as follows: (1) Poly(A), poly(C), and poly(G) sequences are present in all retrovirus RNAs examined. (2) Poly(U) sequences are absent in retrovirus RNA. (3) Poly(C) and poly(G) sequences present in retrovirus RNA are transcribed into complementary poly(dG) sequences and poly(dC) sequences respectively in subunit-length cDNA of retroviruses. (4) The 3'-terminal poly(A) sequences are not transcribed into poly(dT) sequences. (5) Little or no poly(C) and poly(G) sequences have been detected in the cellular nucleic acids examined from nonmalignant mammalian and human embryonic cells. (6) These studies represent a possible molecular approach to cancer prophylaxis and therapy for some types of viral neoplasia, especially human leukemia and cancer of the breast where retrovirus information has been detected.